U.S. patent number 4,424,520 [Application Number 06/311,887] was granted by the patent office on 1984-01-03 for ink jet printing apparatus.
This patent grant is currently assigned to Hitachi Koki Co., Ltd., Hitachi, Ltd.. Invention is credited to Masatoshi Kasahara, Yasumasa Matsuda, Syoji Sagae.
United States Patent |
4,424,520 |
Matsuda , et al. |
January 3, 1984 |
Ink jet printing apparatus
Abstract
An ink jet printing apparatus comprises a nozzle head including,
orifices for ejecting ink particles, pressure chambers each having
a piezoelectric element for applying a pressure wave to ink in the
chamber and each communicating with corresponding one of the
orifices and an ink chamber communicating with the pressure
chambers, and electrical signal applying device for applying an
electrical signal to selected one or ones of the piezoelectric
elements to produce the pressure waves. The electrical signal
applying device applies a main electrical signal pulse to the
selected one or ones of the piezoelectric elements for inducing
rises of the pressure waves and applies a sub-electrical signal
pulse for suppressing the pulsations of the pressure waves a
predetermined time interval after the main electrical signal.
Inventors: |
Matsuda; Yasumasa (Hitachi,
JP), Sagae; Syoji (Hitachiota, JP),
Kasahara; Masatoshi (Hitachi, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
Hitachi Koki Co., Ltd. (Tokyo, JP)
|
Family
ID: |
26333402 |
Appl.
No.: |
06/311,887 |
Filed: |
October 15, 1981 |
Foreign Application Priority Data
|
|
|
|
|
Oct 15, 1980 [JP] |
|
|
55-142991 |
Jan 7, 1981 [JP] |
|
|
56-420 |
|
Current U.S.
Class: |
347/11; 347/48;
347/68 |
Current CPC
Class: |
B41J
2/04533 (20130101); B41J 2/04581 (20130101); B41J
2/055 (20130101); B41J 2/04588 (20130101); B41J
2002/14379 (20130101) |
Current International
Class: |
B41J
2/045 (20060101); B41J 2/055 (20060101); G01D
015/18 () |
Field of
Search: |
;346/14PD |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller, Jr.; George H.
Attorney, Agent or Firm: Antonelli, Terry & Wands
Claims
We claim:
1. An ink jet printing apparatus comprising:
a nozzle head including at least one orifice means for ejecting ink
particles, first ink chamber means having one end thereof
communicated with said orifice means and the other end thereof
communicated with an ink supply aperture to define a pressure
chamber and first piezoelectric transducing means associated with
said first ink chamber means for changing an internal volume of
said first ink chamber means when actuated to eject the ink
particles from said orifice means;
means for selectively applying a set of main and sub-electrical
signal pulses to said first piezoelectric transducing means to
actuate the same, said electrical pulse signal applying means
including first means for generating said main electrical signal
pulse for inducing a rise of a pressure wave in said first ink
chamber means and second means for generating said sub-electrical
signal pulse for suppressing the pulsation of pressure in said
first ink chamber means, said sub-electrical signal pulse having a
phase lagged by a predetermined time interval from said main
electrical pulse signal;
second ink chamber means arranged between said first ink chamber
means and said ink supply aperture and communicating therewith, and
second piezoelectric transducing means associated with said second
ink chamber means for changing an internal volume of said second
chamber means when actuated; and
said set of main and sub-electrical signal pulses further including
a pre-electrical signal pulse to be applied to said second
piezoelectric transducing means to actuate the same, and said
electrical signal applying means includes third means for
generating said pre-electrical signal a predetermined time interval
prior to said main electrical signal pulse.
2. An ink jet printing apparatus according to claim 1, wherein in
the case where a plurality of sets of said orifice means, said
first ink chamber means, said second ink chamber means, said first
piezoelectric transducing means and said second piezoelectric
transducing means are provided, correspondingly respectively, said
main and sub-electrical signal pulses are applied to selected one
or ones of said plural first piezoelectric transducing means and
said pre-electrical signal pulse is applied to the corresponding
one or ones of said plural second piezoelectric transducing
means.
3. An ink jet printing apparatus according to claim 1, wherein in
the case where a plurality sets of said orifice means, said first
ink chamber means and said first piezoelectric transducing means
are provided, correspondingly respectively, only one said second
ink chamber means is provided as second common ink chamber means,
and only one said second piezoelectric transducing means is
associated with said second common ink chamber means, and said main
and sub-electrical signal pulses are applied to selected one or
ones of said first piezoelectric transducing means and said
pre-electrical pulse signal is applied to said only one second
piezoelectric transducing means.
4. An ink jet printing apparatus according to claim 1, 2 or 3,
wherein said electrical signal applying means comprises pulse
signal generating means, first pulse width adjusting means
connected to said pulse signal generating means for adjusting a
pulse width of a pulse signal generated by said pulse signal
generating means to produce said main electrical signal pulse,
delay means connected to said pulse signal generating means for
delaying a phase of said pulse signal generated by said pulse
signal generator by said predetermined delay time, second pulse
width adjusting means for adjusting a pulse width of an output
pulse from said delay means to produce said sub-electrical signal
pulse, advance means connected to said pulse signal generating
means for advancing the phase of said pulse signal generated by
said pulse signal generating means by said predetermined advance
time, and third pulse width adjusting means for adjusting a pulse
width of an output pulse from said advance means to produce said
pre-electrical signal pulse.
5. An ink jet printing apparatus according to claim 4, wherein a
maximum frequency f for driving said nozzle head, namely for
applying the electrical pulse signal set, is selected to satisfy
the following conditions ##EQU4## where B is the number of dots
corresponding to a sum of a width of a unit dot matrix for
representing a character to be printed and a space between two
adjacent unit matrixes, f.sub.1 and f.sub.2 are lowest and highest
frequencies, respectively, of an improper nozzle drive frequency
range, namely an improper frequency range for applying the
electrical pulse signal set, in which proper ink particle is not
formed because of variation of relation between a threshold voltage
of said main electrical pulse signal and a limit voltage for the
formation of proper particle, f.sub.max is an upper limit frequency
of a proper nozzle drive frequency range, namely a proper frequency
range for applying the electrical pulse signal set, in which proper
ink particle is formed at a frequency region higher than f.sub.2,
and n is an integer which meets 0<n.ltoreq.B-1.
6. An ink jet printing apparatus comprising:
a nozzle head including at least one orifice means for ejecting ink
particles, first ink chamber means having one end thereof
communicated with said orifice means and the other end thereof
communicated with an ink supply aperture to define a pressure
chamber and first piezoelectric transducing means associated with
said first ink chamber means for changing an internal volume of
said first ink chamber means when actuated to eject the ink
particles from said orifice means; and
means for selectively applying a set of main and sub-electrical
signal pulses to said first piezoelectric transducing means to
actuate the same, said electrical pulse signal applying means
including first means for generating said main electrical signal
pulse for inducing a rise of a pressure wave in said first ink
chamber means and second means for generating said sub-electrical
signal pulse for suppressing the pulsation or pressure in said
first ink chamber means, said sub-electrical signal pulse having a
phase lagged by a predetermined time interval from said main
electrical pulse signal, said predetermined time interval being set
to be substantially equal to a sum of a pulse width of said main
electrical signal pulse represented in a time unit and a time
required for a pressure wave produced by the application of said
main electrical signal pulse to said first piezoelectric
transducing means, in said first ink chamber means, to reach said
orifice means communicating with said first ink chamber means and
to be reflected back to said first ink chamber means, said
electrical signal applying means comprises pulse signal generating
means, first pulse width adjusting means connected to said pulse
signal generating means for adjusting a pulse width of a pulse
signal generating by said pulse signal generating means to produce
said main electrical signal pulse, delay means connected to said
pulse signal generating means for delaying a phase of said pulse
signal generated by said pulse signal generator by said
predetermined delay time, and second pulse width adjusting means
for adjusting a pulse width of an output pulse from said delay
means to produce said sub-electrical signal pulse, and wherein a
maximum frequency f for driving said nozzle head, namely for
applying the electrical pulse signal set, is selected to satisfy
the following conditions ##EQU5## where B is the number of dots
corresponding to a sum of a width of a unit dot matrix for
representing a character to be printed and a space between two
adjacent unit matrixes, f.sub.1 and f.sub.2 are lowest and highest
frequencies, respectively, of an improper nozzle drive frequency
range, namely an improper frequency range for applying the
electrical pulse signal set, in which proper ink particle is not
formed because of variation of relation between a threshold voltage
of said main electrical pulse signal and a limit voltage for the
formation of proper particle, f.sub.max is an upper limit frequency
of a proper nozzle drive frequency range, namely a proper frequency
range for applying the electrical pulse signal set, in which proper
ink particle is formed at a frequency region higher than f.sub.2,
and n is a positive integer which meets 0<n.ltoreq.B-1.
7. An ink jet printing apparatus according to claim 6, wherein said
electrical pulse signal applying means applies the electrical
signal pulses over a large frequency range to said first
piezoelectric transducing means to actuate the same and enable
injection of the ink particles from said orifice means, said first
generating means generating said main electrical signal pulse with
a threshold value necessary for inducing a rise of a pressure wave
in said first ink chamber means, and said second generating means
generating said sub-electrical signal pulse phase lagged by a
predetermined time interval from said main electrical signal pulse
for suppressing the pulsation of pressure in said ink chamber means
so as to enable the threshold value versus frequency characteristic
of the ink jet printing apparatus to be substantially flat over a
large frequency range.
Description
The present invention relates to an ink jet printing apparatus, and
more particularly to an ink jet printing apparatus in which an
internal volume of an ink chamber formed in a nozzle head is varied
to eject ink particles from an orifice.
The ink jet printing apparatus of this type is known, as disclosed,
for example, in U.S. Pat. No. 4,216,477 to Matsuda et al issued on
Aug. 5, 1980, as an impulse jet system which comprises an ink
chamber having one end communicated with an ink tank and the other
end communicated with an orifice for ejecting ink particles to form
a pressure chamber, and a nozzle head having an electromechanical
transducer such as a piezoelectric crystal or element which forms a
portion of a wall of the ink chamber and abruptly reduces a volume
of the ink chamber upon application of an electrical pulse signal
so that the volume of the ink chamber is varied by the electrical
signal applied to the piezoelectric element crystal or to eject the
ink in the ink chamber from the orifice one ink particle at a time
in synchronism with the electrical signal to record a desired
pattern on a recording paper.
In the impulse jet system, one ink particle is ejected from the
orifice for each electrical signal applied to the piezoelectric
element. Accordingly, a recording speed of the impulse jet system
is lower than other systems but it has been recognized as a simple
type recording apparatus because the structure of the nozzle head
is simple and neither means for recovering unused ink particles nor
control means for the ink particles is required.
In this type of printing apparatus, since the ink particles are
ejected from the orifice by abruptly reducing the internal volume
of the ink chamber by applying the electrical signal to the
piezoelectric element mounted in the ink chamber, various problems
may arise in the selection of a waveform, a pulse width, an
amplitude (voltage) and a frequency of the electrical pulse
signal.
The pulse waveform is preferably a square wave from the standpoint
of abruptly reducing the internal volume of the ink chamber, and
more preferably it has a sharp rise. In an experiment, it has been
found that the rise dV/dt is preferably larger than
2.5.times.10.sup.8 volts/second.
It will be readily understood that the pulse width is preferably
within a predetermined range in order to produce an ideal ink
particle. In an experiment, it has been found that the pulse width
is preferably within a range of 20-80 microseconds.
It has also been found that the magnitude (voltage) of the
electrical pulse signal may give a significant effect on the
formation of the ink particle. When the voltage of the electrical
pulse signal is too low, a pressure pulse large enough to overcome
a surface tension of liquid in the orifice can not be produced and
hence no ink particle is ejected. A minimum voltage necessary for
the formation of a proper ink particle is referred to as a
threshold voltage hereinafter. When an electrical pulse signal
larger than the threshold voltage is applied, the size of the ink
particle ejected from the orifice and its flying speed increase in
proportion to the voltage. However, if the voltage of the
electrical pulse signal increases beyond a certain critical value,
the proper ink particle formation can not be attained, so that a
large ink particle together with a very fine ink particle are
formed or the large ink particle is not formed but only a plurality
of small ink particles are formed. The upper limit of the voltage
which allows the proper ink particle formation is referred to as a
proper particle formation limit voltage.
The relationship between the frequency of the electrical pulse
signal applied to the piezoelectric element and each of the
threshold voltage and the proper particle formation limit voltage
becomes also a problem while it is not critical when the ink
particles are ejected only around a particular frequency (e.g. 1000
Hz), a usual printing apparatus is driven at any desired frequency
and hence it is required that the ink particles are properly
ejected over a wide frequency range. In such a printing apparatus,
it is preferable in the design of an electric drive circuit that
the threshold voltage and the proper particle formation limit
voltage are substantially constant over a wide frequency range and
the former is as low as possible.
Considering first the frequency characteristic of the threshold
voltage, in the prior art apparatus, the variation of the threshold
voltage increases with the increase of the frequency, and in order
to form the ink particle having a uniform size and a uniform
velocity over a wide frequency range, the magnitude of the
electrical pulse has to be adjusted for each frequency used.
Accordingly, the circuit configuration is complex and expensive, or
the use of the apparatus at a high frequency has to be given
up.
It has been found that, while the frequency characteristic of the
threshold voltage of the prior art nozzle head of the type
described above is affected by a mechanical resonance of the nozzle
head, a part of the variation of the frequency characteristic of
the threshold voltage can not be improved even if the mechanical
resonance point is varied, that is, there exists a variation factor
other than the mechanical resonance.
This variation factor is inherent to the nozzle head and it has
been found by an experiment that it is caused by the fact that the
pulsation of the pressure due to a fluidic resonance of the ink in
the nozzle head renders the pressure change produced in the ink
chamber by the electrical signal applied to the piezoelectric
element to be frequency-dependent.
It has also been found that regarding the fluidic resonance per se
in the fluid path in the nozzle head, the pulsation of the pressure
in the ink chamber when the phase of the pressure wave which has
emanated from the ink chamber, reached the orifice at the tip end
of the nozzle head and reflected back to the ink chamber and the
phase of the pressure change due to the deformation of the
piezoelectric element in the ink chamber are in phase with each
other, directly affects the formation of the ink particle even if
the resonance frequency is much higher than the drive
frequency.
Furthermore, not only the frequency characteristic of the threshold
voltage but also the frequency characteristic of the proper
particle formation limit voltage is critical. While the proper
particle formation limit voltage also varies over a wide frequency
range, it does not necessarily vary with the variation of the
threshold voltage but both the voltages may be very close to each
other at a certain frequency or the threshold voltage may so rise
at another frequency that it becomes equal to the proper particle
formation limit voltage. The frequency at which the threshold
voltage abnormally rises is a frequency limit. In the prior art, a
response frequency range has been set such that the maximum
threshold voltage does not exceed the minimum level of the proper
particle formation limit voltage. If the frequency limit is low,
the print speed of the ink jet printer is necessarily low.
Accordingly, in order to improve the performance of the ink jet
printing apparatus, it has been desired to raise the frequency
limit to broden the response frequency range.
It is, therefore, an object of the present invention to provide an
ink jet printing apparatus which overcomes the difficulties
described above relating to the electrical pulse signal applied to
the piezoelectric element.
It is another object of the present invention to provide a high
performance ink jet printing apparatus capable of forming
substantially uniform ink particles over a wide frequency
range.
It is a further object of the present invention to provide an ink
jet printing apparatus which cancels out a pulsation of a pressure
of ink in an ink chamber of a nozzle head due to a reflected wave
to reduce the effect of the pulsation of the pressure to the ink
particle.
It is a further object of the present invention to provide an ink
jet printing apparatus which suppresses the variation of the
threshold voltage with respect to the frequency characteristic of
the nozzle head and reduces the threshold voltage to improve the
controlability of the apparatus.
It is still further object of the present invention to provide an
ink jet printing apparatus having a high printing speed.
The above and other objects and features of the present invention
will be apparent from the following description taken in
conjunction with the accompanying drawings, in which:
FIG. 1 is a plan view, partly cut away, of a nozzle head in one
embodiment of the present invention;
FIG. 2 is a sectional view taken along a line II--II in FIG. 1;
FIG. 3 diagramatically shows an electrical pulse signal and a
pressure change in an ink chamber;
FIG. 4 shows a waveform of a pair of main and sub-pulses used to
drive a piezoelectric element in an embodiment of the present
invention;
FIG. 5 shows frequency characteristics of the threshold voltage in
various cases;
FIG. 6 is a block diagram of an information signal source circuit
for driving the nozzle head shown in FIG. 1;
FIG. 7 is a plan view, partly cut away, of a nozzle head in another
embodiment of the present invention;
FIG. 8 is a block diagram of an information signal source circuit
for driving the nozzle head shown in FIG. 7;
FIG. 9 shows an example of characters printed by a print head
according to the present invention; and
FIG. 10 is a diagram illustrating a relationship between the
driving frequency for the print head and each of the threshold
voltage and the proper particle formation limit voltage.
The preferred embodiments of the ink jet printing apparatus
according to the present invention will now be described in detail
with reference to the accompanying drawings.
FIG. 1 is a plan view, partly cut away, of a nozzle head in one
embodiment of the present invention. A substrate 2 of a nozzle head
generally designated by 1 has five ink chambers 3a-3e which form
independent pressure chambers, orifices 4a-4e communicating with
respective end surfaces of the ink chambers 3a-3e, a common ink
chamber 5 and fluid path grooves 7a-7e extending between the common
ink chamber 5 and the respective ink chambers 3a-3e and having
fluidic diodes 6a-6e, respectively.
The common ink chamber 5 communicates with an ink tank 10 through
an ink supply aperture 8 and a pipe 9.
An upper cover 11 is joined to the substrate 2 thus constructed, as
shown in FIG. 2, and piezoelectric elements 12a-12e are fixedly
bonded to the upper surface of the upper cover 11 at positions
corresponding to the ink chambers 3a-3e, respectively.
Ink 13 in the ink tank 10 is supplied to the ink chambers 3a-3e
through the ink supply aperture 8, the common ink chamber 5, and
the fluidic diodes 6a-6e and it is filled up to the orifices 4a-4e
which are connected to the ink chambers 3a-3e, respectively.
When an electrical signal is applied from an information signal
source 14 to selected one or ones of the piezoelectric elements
12a-12e in a polarity to cause internal volumes of corresponding
one or ones of the ink chambers 3a-3e to be reduced, the pressures
in the corresponding one or ones of the ink chambers 3a-3e rise so
that ink particles 15 are ejected from the corresponding one or
ones of the orifices 4a-4e toward a record medium 16.
The fluidic diodes 6a-6e formed in the fluid path grooves 7a-7e
between the common ink chamber 5 and the separate ink chambers
3a-3e function to minimize the propagation of the pressures of the
ink 13 produced in the corresponding one or ones of the ink
chambers 3a-3e to the common ink chamber 5 so as to maximize the
propagation of the pressures to the corresponding one or ones of
the orifices 4a-4e.
In the ink jet printing apparatus thus constructed, the pressure
changes with respect to the electrical signal in the corresponding
one or ones of the ink chambers 3a-3e are shown in FIG. 3.
When the electrical signal as shown in FIG. 3(a) is applied to a
selected one of the piezoelectric elements 12a-12e, the pressure in
the corresponding one of the ink chambers 3a-3e changes as shown in
FIG. 3(b).
At a rise T.sub.1 of the pulse, the internal volume of the
corresponding ink chamber is abruptly reduced to raise the pressure
in the corresponding ink chamber. The resulting pressure wave is
propagated to the orifice connected to the corresponding ink
chamber and the internal pressure of the corresponding ink chamber
is immediately recovered.
When the electrical pulse signal terminates at T.sub.2, the
internal volume of the ink chamber now abruptly expands so that the
internal pressure reaches a negative pressure, that is, a pressure
lower than an atmospheric pressure.
When a positive pressure, that is, a pressure higher than the
atomospheric pressure in the selectively actuated ink chamber
reaches the orifice connected to the ink chamber, the ink particle
15 is ejected from the orifice and the pressure wave decays.
Thereafter, the ink 13 in the orifice is instantly pulled into the
nozzle head 1 by the negative pressure and the pressure wave is
reflected back into the ink chamber.
For example in Japanese Patent Application Laid-open No.
109935/1977 filed Dec. 7, 1976 and laid-open Sept. 14, 1977 and
Japanese Patent Application Laid-open No. 64230/1977 filed Nov. 15,
1976 and laid-open May 27, 1977, the amount of pull-in of the ink
into the nozzle head by the negative pressure is reduced to prevent
the formation of small ink particle and the deflection of flying
direction of the ink particle. However, it has been provided by an
experiment that it is not effective to improve the frequency
characteristic of the threshold voltage which the present invention
intends. It is considered that the threshold voltage varies with
the frequency because the reflected wave goes back to the ink
chamber to cause the pulsation of the pressure in the ink chamber
and the phase of the pulsation of the pressure in the ink chamber
and the phase of the rise of the pressure pulse by the drive pulse
are displaced with the frequency. Since the apparatus disclosed in
the above-mentioned laid-open patent applications are not effective
to prevent the pulsation, they are not effective to improve the
frequency characteristic of the threshold voltage.
In accordance with one embodiment of the present invention, as
shown in FIG. 4, a second or sub-pulse signal P.sub.2 is applied to
the selected piezoelectric element a predetermined time interval
after a first or main pulse P.sub.1, in a polarity to cancel out
the pressure pulsation due to the reflected wave so that the
variation of the pressure in the ink chamber is reduced to thereby
improve the frequency characteristic of the threshold voltage. The
main pulse P.sub.1 induces the rise of the internal pressure of the
ink chamber and the sub-pulse P.sub.2 applied .DELTA.T after the
main pulse P.sub.1 suppresses the pressure pulsation due to the
reflected negative pressure wave.
By properly selecting the parameters such as a voltage V.sub.1 and
a pulse width W.sub.1 of the main pulse P.sub.1, a voltage V.sub.2
and a pulse width W.sub.2 of the sub-pulse P.sub.2 and the delay
time .DELTA.T, the frequency characteristic of the nozzle head is
improved as will be described below.
FIG. 5 is for explaining the frequency characteristics of the
nozzle head 1 in the case where a pair of main and sub-electrical
pulse signal P.sub.1 and P.sub.2 are applied to a selected
piezo-electric element. According to the embodiment of the
invention and in the case where a single pulse signal is applied as
in the conventional technique.
In the case where a single electrical pulse signal is applied to
the nozzle head 1, the threshold voltage significantly varies with
the frequency as shown by a solid line curve (I), and in the case
where a set of the main pulse P.sub.1 and the sub-pulse P.sub.2 are
applied with the delay time .DELTA.T being equal to 120
microseconds, the variation of the threshold voltage with the
frequency is small as shown by a dot-and-dash line curve (II) and,
thus, the threshold voltage versus frequency characteristic is
substantially flat and stable over a large frequency range. A
broken line curve (III) shows the frequency characteristic in the
case where the delay time .DELTA.T is selected to be 50
microseconds. In the last case, it has been found that the
variation of the threshold voltage is rather larger than that in
the conventional case where a single pulse is applied. This is
because the sub-pulse signal P.sub.2 is applied before the pressure
wave caused by the main pulse signal P.sub.1 and reflected back
from the orifice has reached the ink chamber again so that the
pulsation due to the reflected wave by the main pulse signal
P.sub.1 and the pulsation due to the reflected wave by the
sub-pulse signal P.sub.2 are always produced in the ink chamber and
they adversely affect the pressure change in the ink chamber.
Accordingly, it is necessary to set the delay time .DELTA.T between
the pair of main and sub-electrical pulse signals applied to the
selected piezoelectric element to a proper duration. It has been
found that by selecting the delay time .DELTA.T to
where .DELTA.t.sub.1 is the pulse width (represented in time) of
the main electrical pulse signal P.sub.1, and .DELTA.t.sub.2 is a
propagation time of the pressure from the ink chamber to the
orifice and therefore expressed by .DELTA.t.sub.2 =L/C, L being an
effective length from the ink chamber to the orifice, C being a
sound velocity in the ink, the pulsation of the pressure in the ink
chamber due to the reflected wave by the main electrical pulse
signal P.sub.1 is cancelled by the sub-electrical pulse signal
P.sub.2 so that a stabilized ink particle is ejected from the
orifice.
In the present embodiment the ink particle of proper size can be
ejected at a frequency of 5000 Hz or higher, while in the prior art
single pulse system, the frequency limit for the ejection of the
ink particle without adjusting the magnitude of the electrical
pulse signal is 3000 Hz.
FIG. 6 shows a block diagram of an information signal source
circuit for driving the nozzle head. An output pulse P.sub.01 from
a signal pulse generator 21 is applied to a pulse width adjuster 24
which produces the main pulse P.sub.1 having a pulse width W.sub.1.
The output pulse P.sub.01 of the pulse generator 21 is also
supplied to a delay circuit 22 which produces a pulse P.sub.02
which is delayed by .DELTA.T from the pulse P.sub.01. The pulse
P.sub.02 is supplied to a pulse width adjuster 25 which produces
the sub-pulse P.sub.2 having a pulse width W.sub.2. The output
pulses P.sub.1 and P.sub.2 from the pulse width adjusters 24 and 25
are combined in an adder 27 and the combined pulse signal is
applied to selected one or ones of the piezoelectric elements
12a-12e through an amplifier 28. A relation of the pulse width
W.sub.2 of the sub-pulse P.sub.2 to the pulse width W.sub.1 of the
main pulse P.sub.1 is experimentarily determined. In the present
embodiment, the voltages V.sub.1 and V.sub.2 of the main pulse
P.sub.1 and the sub-pulse P.sub.2 are equal. Alternatively, an
amplifier may be inserted at a point A, B or C in the sub-pulse
generation circuit so that the voltage V.sub.2 of the sub-pulse
P.sub.2 is changed relative to the voltage V.sub.1 of the main
pulse P.sub.1.
FIG. 7 is a plan view, partly cut away, of a nozzle head in
accordance with another embodiment of the present invention. The
like numerals to those shown in FIG. 1 denote the like elements and
hence they are not explained here. Although only a set of orifice,
ink chamber and piezoelectric element is shown for the purpose of
simplification, the other sets may be constructed in the same
manner. Between the ink supply aperture 8 of the nozzle head 1 and
the first ink chamber 3a (3b-3e), a second ink chamber 17a
(17b-17e) is formed to define a second pressure chamber in series
with the first ink chamber 3a (3b-3e) and a second piezoelectric
element 18a (18b-18c) is joined on the upper surface of the upper
cover 11 at the position corresponding to the second ink chamber
17a (17b-17e).
A pre-electrical pulse signal P.sub.3 which preceeds to the main
electrical pulse P.sub.1 applied to the piezoelectric element 12a
(12b-12e) corresponding to the first ink chamber 3a (3b-3e) for
injecting the ink particle 15, by a predetermined time interval
.DELTA.T', is applied to the second piezoelectric element 18a
(18b-18e) corresponding to the second ink chamber 17a
(17b-17e).
FIG. 8 shows a block diagram of an information signal source
circuit for driving the nozzle head shown in FIG. 7. In this
circuit, a pre-pulse signal generating circuit is added to the main
and sub-pulse signal circuit shown in FIG. 6. The main pulse
P.sub.1 and the sub-pulse P.sub.2 are generated in the same manner
as shown in FIG. 6, and the like numerals denote the like
elements.
The additional pre-pulse signal generating circuit includes a pulse
advance circuit 23, a pulse width adjuster 26 and an amplifier 29.
The output pulse P.sub.01 from the pulse generator 21 is supplied
to the pulse advance circuit 23 which produces a pulse P.sub.03
advanced by .DELTA.T' from the pulse P.sub.01. The pulse P.sub.03
is supplied to the pulse width adjuster 26 which produces the
pre-pulse P.sub.3 having a pulse width W.sub.3 which in turn is
applied to the second piezoelectric element 18a (18b-18e) of the
second ink chamber 17a (17b-17e) through the amplifier 29. A
voltage V.sub.3 of the pre-pulse P.sub.3 may be varied by the
amplifier 29.
In the apparatus of the present embodiment, when the pre-pulse
signal P.sub.3 is applied to the piezoelectric element 18a
(18b-18e) of the second ink chamber 17a (17b-17e), a pressure wave
is produced in the second ink chamber 17a (17b-17e). By applying
the main pulse signal P.sub.1 to the piezoelectric element 12a
(12b-12e) of the first ink chamber 3a (3b-3e) when a wave front of
the pressure wave reaches the first ink chamber 3a (3b-3e), the
rise of the pressure in the ink chamber 3a (3b-3e) is rendered
sharp. As a result, the magnitude of the main pulse signal P.sub.1
applied to the piezoelectric element 12a (12b-12e) of the first ink
chamber 3a (3b-3e), and hence the threshold voltage for ejecting
the ink particle 15 from the orifice 4a (4b-4e) may be lowered.
Since the pressure change in the second ink chamber 17a (17b-17e)
is superimposed on the pressure change in the first ink chamber 3a
(3b-3e), the pulsation in the ink chamber 3a (3b-3e) is enhanced.
This pulsation, however, can be suppressed by applying the subpulse
signal P.sub.2 to the piezoelectric element 12a (12b-12e) of the
first ink chamber 3a (3b-3e). Thus, as a whole, the nozzle head
having a low threshold voltage and less pulsation of the pressure
in the ink chamber can be provided.
Since the electrical signal applied to the piezoelectric element
may be low, the control is facilitated, and the printing apparatus
having a low threshold voltage for injecting the ink particle and
capable of forming a uniform ink particle over a wide frequency
range can be provided.
The pulse width W.sub.3, the voltage V.sub.3 and the advance time
.DELTA.T' of the pre-pulse signal P.sub.3 can be experimentarily
determined.
While the plurality of second ink chambers 17a-17e are arranged in
series with the first ink chambers 3a-3e, respectively, which
inject the ink particles in the present embodiment, the second ink
chambers may be a common ink chamber like in the first embodiment
and a single piezoelectric element may be arranged in the common
ink chamber so that an initial pressure wave is transmitted
therefrom to the respective ink chambers. In this modification,
there is no need to provide separate second ink chambers and hence
the structure of the nozzle head is simplified.
As is well known, when characters or symbols are printed by the ink
jet printing apparatus having the nozzle head constructed as shown
in FIG. 1, the ink particles are selectively ejected from the
vertically arranged orifices 4a-4e of the nozzle head 1 while the
head is laterally moved to serially print out the characters, as
shown in FIG. 9. While five orifices are shown in FIG. 1, seven
orifices may be used to define a seven-dot column as seen in FIG.
9. By laterally shifting the head five times, a 7.times.5 dot
matrix character or symbol can be printed out. Generally, in the
7.times.5 dot matrix system, a two dot space is inserted between
every two adjacent characters or symbols.
When the respective numbers of dots in vertical and horizontal
directions of the matrix are given by A and B, respectively, the
horizontal space is given by B' dots, the printing speed is given
by C characters per second, and the drive frequency for the nozzle
head is given by f cycles, the following relation is met:
where
FIG. 10 shows a chart of the threshold voltage versus the frequency
of the proper particle formation limit voltage. A solid line curve
P shows a frequency characteristic of the threshold voltage when a
set of main and sub-pulses are applied in accordance with the
embodiment of the present invention, and a curve Q shows a
frequency characteristic of the proper particle formation limit
voltage. A dot-and-dash line curve P' shows a frequency
characteristic of the threshold voltage when a single pulse in the
prior art system is applied and a curve Q' shows the frequency
characteristic of the proper particle formation limit voltage.
As seen from FIG. 10, the variations of the threshold voltage P and
the proper particle formation limit voltage Q in the present
embodiment are less than those of P' and Q' in the prior art
system. This trend is particularly remarkable in the frequency
characteristic of the threshold voltage. Also as seen from FIG. 10,
the response frequency limit f.sub.b in the present embodiment is
higher than the response frequency limit f.sub.a in the prior art
single pulse system, and a frequency range R for the proper
particle formation limit voltage exist in a high frequency region.
However, while the response frequency limit is expanded by the
double pulse drive in accordance with the present embodiment, the
threshold voltage and the proper particle formation limit voltage
significantly vary in a certain frequency region (around 2 KHz) as
shown in FIG. 10. It has been found that if the frequency range of
the proper particle formation limit voltage is in a high frequency
region in which the nozzle head drive frequency f meets the
relations described below, the high frequency region may be used to
drive the nozzle head. ##EQU1## where f.sub.1 and f.sub.2 are
lowest and highest frequencies, respectively, of a frequency region
in which there is no frequency range of the proper particle
formation limit voltage, f.sub.max is an upper limit frequency of
the frequency range R of the proper particle formation limit
voltage, and n is a positive integer where
If the frequency f which meets the above requirements is attained,
the maximum operating frequency can be determined within that
frequency range of the frequency f.
A specific example of the frequencies used in the high speed ink
jet printer in the present embodiment is now explained with
reference to FIG. 10. In the case where the frequency
characteristics of the threshold voltage and the proper particle
formation limit voltage are substantially constant under 2000 Hz so
as to allow a frequency range of proper particle formation to
exist, they significantly vary between 2000-2300 Hz so as to allow
no frequency range of proper particle formation to exist, and they
are again stabilized above 2300 Hz so as to allow another frequency
range of proper particle formation to exist, the frequencies
f.sub.1 and f.sub.2 are given by 2100 Hz and 2300 Hz, respectively.
When the character is printed out by 7.times.5 dot matrix with the
space of two dots, B=5, B'=2 and hence B=5+2=7. Accordingly, the
relations (3), (4) and (5) are represented as follows: ##EQU2##
From the formulas (7) and (8), ##EQU3## From the formula (6),
From the formula (7),
when n=4, 3675>f>3220
when n=5, 2940>f>2683
when n=6, 2450>f>2300
In the present embodiment, the nozzle head drive frequency f is
selected within those ranges as mentioned above. Accordingly, the
affect of the frequency range from f.sub.1 (2100 Hz) to f.sub.2
(2300 Hz) is avoided and a high drive frequency can be selected to
attain a stable and high speed print characteristic. When more than
one poor frequency characteristic ranges exist in the frequency
region lower than f.sub.1, the proper frequency f is determined for
each of the ranges and if a frequency region common to all of the
frequencies f, the drive frequency can be set in the high frequency
region without being affected by the plurality of poor frequency
characteristic regions.
While the liquid injected from the nozzle head is ink and it is
used to print the characters in the illustrated embodiments, the
present invention is not limited to such specific embodiments but
any liquid which can be formed into particles may be used, and it
may be used for measurement or analysis. For example, a digital
controlled micropipet for placing a small quantity of liquid into a
vessel may be constructed.
* * * * *